Lung cancer is the leading cause of cancer deaths both in the United States and worldwide. Despite many classification schemes and ongoing clinical trials, there has been overall disappointing progress in the field of clinical diagnostics and therapeutics. Approximately 172,000 tumors of the lung were diagnosed in 2005 with an estimated 163,000 deaths, more than colon, breast, and prostate combined. At least 75% of patients present with locally advanced disease. Although there has been much effort to improve screening using technology such as high-resolution CT, these methods often produce false positive results and usually do not change outcome. Thus, even small tumors detected early present a significant threat to patients with postoperative 5-year survival rates for stage I lung cancer estimated between 47 to 63 percent. For patients with advanced disease the prognosis is worse with median survivals well under a year. In general, palliative therapy is effective but not sustainable and the average impact on overall survival is approximately 3 months. At the population level the underlying cause of lung cancer is clearly tobacco use, with 90% of all lung cancers attributed directly to smoking. Smoking is so tightly correlated with lung cancer that it confounds definitive association with most other risk factors; although asbestos, radon, and a number of lung irritants are generally accepted as lung cancer risk factors. A genetic association is strongly suspected, however, the exact mechanism remains to be determined outside of a select group of rare Mendelian cancer syndromes.